Key Questions in Oscar Pistorius Murder Trial

The shooting death of model Reeva Steenkamp by "Blade Runner" Oscar Pistorius has captured global attention and led to a blot of unresolved questions, but those questions will begin to be answered Monday when Pistorius goes on trial on a charge of premeditated murder.

Prosecutors in Pretoria, South Africa, say Pistorius, 27, intentionally shot Steenkamp, 29, in the bathroom of his home in a gated community on Valentine's Day in 2013 during an argument. Pistorius also faces three firearms charges.

The athlete, known as the "Blade Runner," does not deny that he fired four shots through a locked bathroom door, hitting Steenkamp three times. But he insists he acted in self-defense after mistaking her for an intruder during the middle of the night.

The two sides have listed more than 100 witnesses including family members, ex-girlfriends, neighbors, police, forensic psychologist, ballistic experts, even a weather forecaster. The trial could last up to six weeks, officials said.

If convicted, Pistorius could spend the rest of his life in prison.

Here's a look back at how the world-renowned runner went from the respected "Blade Runner" to suspected "Blade Gunner," and a preview of how both sides plan to make their case.

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Pistorius Runs Onto the World Stage

Born with congenital absence of the fibula in both legs, Pistorius had both legs amputated when he was 11 years old.

Nicknamed the "Blade Runner" for the carbon fiber blades he runs on, Pistorius stepped onto the world stage when he was chosen to represent South Africa at the 2012 Olympic and Paralympic games in London.

The "fastest man with no legs" finished last in his 400 meter semi-final and was eliminated, but the story of overcoming his disability captivated the world and led to lucrative endorsement deals.

Pistorius has argued in court that he was closing his balcony doors Feb. 14, 2013 when he heard a noise from the bathroom. Fearing an intruder, and without his prosthetic legs on, he grabbed a gun from under his bed and fired through the closed bathroom door, he told the court.

Prosecutors initially said they believed Pistorius had put on his artificial legs before shooting Steenkamp, showing intent.

But in a move that may bolster Pistorius' defense, the state's ballistics expert recently backed Pistorius' claim that he was walking on his stumps when he shot Steenkamp, according to leaked documents obtained by South African news network eNCA.

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What the Neighbors Heard

Prosecutors allege the athlete shot Steenkamp during an argument, however it is unclear what the couple may have been arguing about or a possible motive, South African media reported.

Lawyers said neighbors reported hearing shouts from Pistorius' home the night of the shooting.

When asked about the witness who allegedly heard yelling between Pistorius and Steenkamp, police officer Hilton Botha, who was one of the first to arrive at the scene, admitted under cross-examination that the woman was about 600 yards -- six football fields -- away at the time.

Pistorius cotnends that he and Steenkamp had gone to bed, falling asleep hours before the shooting took place.

Another key to the prosecution's case: Police said no calls to the police or ambulance service were made on any of the four cellphones -- two iPhones and two BlackBerrys -- found in the bathroom and bedroom of Pistorius' home. Investigators said that guards at the gated estate called Pistorius, who told them he was "all right." They could hear him crying, police said.

Pistorius says he called the manager of the housing estate, and asked him to place a call for an ambulance. He says that he also called a private paramedic service. According to his lawyers, there was a fifth phone that Pistorius used to make the calls.

Pistorius' Locked Phone

The lead detective on Pistorius' case traveled from South Africa to the Apple headquarters in California to ask for help in bypassing password protection on the athlete's iPhone, the Telegraph reported.

The phone, which was reportedly found lying outside the bathroom where Steenkamp was shot, could provide prosecutors with clues about a possible motive and the sequence of events that led up the shooting.

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Blade Runner to 'Blade Gunner'

Pistorius, who has been out on bail since his arrest last February, released a statement on the eve of the one year anniversary of the shooting, saying "the loss of Reeva and the complete trauma of that day, I will carry with me for the rest of my life."

Steenkamp's parents, June and Barry Steenkamp, sent out their own statement saying some of the the family plans to attend Pistorius' trial. No one from the Steenkamp family was present during Pistorius' earlier court appearances.

Prosecutors are seeking a life sentence for Pistorius, meaning if he is convicted, he will have to serve at least 25 years in prison before being eligible for parole.

It was not known whether Pistorius planned to testify in his defense. Parts of the court proceedings, which has been billed as South Africa's "trial of the century," will be televised.